Wall-stabilized electric high-pressure gaseous discharge lamp



Feb. 9, 1960;

WALL-STABILIZED ELECTRIC HIGH-PRESSURE GASEOUS DISCHARGE LAMP 1&1.

H. SCHIRMER ET AL Filed Sept. 11, '1958 Hovs l: Grabner, b

Their ACL'CIB'OT'TWH.

United States PatentO WALL-STABILIZED ELECTRIC HIGH-PRESSURE GASEOUSDISCHARGE LAMP I-Ierbert Schiriner, herlin flharlottenburg, and HorstGrabner, Berlin-Zehlendorf, Germany, assignors toPate'nt-Treuhand-Gesellschaft fur elektrische Giulilampen m.b.H.

Application September 11, 1958, Serial No. 760,404

(Elaims priority,- application Germany September- 17, 1957 8 Claims.(Cl; 313-185) This invention relates to high-pressure gaseous dischare-l'amps fer continuous operation; as distinguished from" pulsedoperation: It relates more particularly to Xenon discharge lamps,wherein the discharge are is stabilized' by the influence of the wall'ofthe discharge envelope and" the electrode distance amounts to a multipleof the envelope diameter.

An arc discharge may be" designated as wall stabilized if noconvectioneifectsappear. In such case the arc fills the whole cross section of thedischarge envelope up to" an edge zone determined by decrease oftemperature ofthe plasma towards the't'ube wall; The data relative tothe arc may thenb'e'" calculated by integration of'the Elenbass-Hellerdifferential e uation and with the aid of the theoryofelectricalconductivity, heat conductivity and radiation".

In practical realizations of wall stahilized high-pressure gaseous longare lamps, more particularly xenon lamps, for continuousop'eratior'ithere have beenused'up to now, for the purpose of obt'aininga sufiiciently-' high gas temperature for 'high luminus efiiciency, veryhigh high input concentrations in plasma and wall loading. These havebeen-so high that only by means of artificial cooling, e.g., watercooling, could melting of the" quartz glass used as the envelopematerial be prevented; Such xenon high-pressure gaseous discharge lampsare designatedcommercially as type XBF lamps; they show an efficiency of35 lm./wl- (lu'mens per watt) and burn silently. They'may be builtalsoas lamps'of high wattage; The required wat'er cooling, however,increases the cost of such lamps and restricts their utilization;

High-pressure gaseous discharge lamps as" known hitherto which do notrequire any artificial cooling but only cooling by natural airconvection have, up to the present, never been made as wall-stablizedlamps" of low filling or operating pressure. There are known forinstance air-cooled short-arc lamps with a large bulb and, comparedtherewith; a very small arc diameter and in consequence thereof withhighpower concentration; because of the limited temperatureresistance ofthe quartz glass it is impossible with such high power concentration toprovide the bulb wall .a'snear to the are as is necessary for obtaininga stabilizing eiie'ct. There is also well know an elonagted xenonhigh-pressure discharge lamp without artificial cooling but with loweroutput, e.g 1 kilowatt, in which the arc fills only a small part of theenvelope cross section; it cannot be considered as wallstabilized in thepresent sense but it has a convectional operation. Its cold pressure isproportionately high with 600 mm. Hg. In another design of an air-cooledxenon lamp stabilizing of the are is obtained by several diaphragms eachwith a central aperture in the discharge path. In other casesstabilizing is obtained by externally applied magnetic fields. All theselamps without artificial cooling have; hitherto, been provided with agas filling of proportionately high pressure in order to obtain a highelectrical wattage per unit of volume and, thereby, as high atemperature and light output as possible.

High-pressure gaseous discharge lamps according to the presentinvention, the electrode distance of which amounts at least to doublethe inside tube diameter are wall-stabilized. This has been establishedtheoretically and confirmed experimentally. Artificial cooling of thedischarge tube being omitted, the lamps have an avearge inputconcentration in the discharge which lies between about 5 and 200 w./cm.(watts per cubic centimeter) only if quartz glass is used as thebulb'material and have a filling pressure-of the gas or gases reduced toa lamp without dead space between S and 350 mm. Hg.

The power concentration is lower by at least one order of magnitude(factor'of 10) than with known artificially cooled lamps. This factresults from the lower heat radiationi bythe bulb surface in consequenceof omitting any artificial cooling; An important feature in such lampsaccording to this invention consists in that with the required low inputconcentration, the pressure must be kept low so that the arc isnotdisturbed by convectio'n eifects but is Wallstabilized. Also the lowpressure r permits sufiiciently high gas temperatures to be obtained.

The importance of this fact may be understood when it 18 considered thatthe radiation power of an arc (with maintained temperature) decreases asthe pressure is reduced because the number of radiating atoms decreaseswith reduced pressure at constant temperature. Thus the arc= must have ahigher temperature for an applied electrical wattage at low pressurethanat higher pressure provided the and diameter is so dimensioned that theincreasedheat conduction at'low pressure does not make completelyineffective any temperature increase. Quant tativ'e calculations haveindicated these functional relations betweeninput concentration,filling-pressure, radiation power and-heat losses whereas simplequalitative considerations have been unproductive of exact conclusions.

When such low filling pressures are used in the field off high-pressuregaseous discharge, e.g., with xenon, it might be feared that a thermalbalance would no longer be achieved so that the discharge would not showany strong continuum andsufii'cient light output. This suppositionisrather natural because of the small cross sectrons of action of xenonatoms in regards to probability of an electron impact. In fact, however,a thermal balance will be achieved with these pressures if suflicientlyhigh current intensities" are applied. A reason for this latter fact maybe seen in the extensive coulomb fields of the ions which bring aboutthe necessary coupling of the electron gas and the carrier gas.

Lamps according to the present invention with low input concentrationare discharge lamps having real high-pressure characteristics, asevidenced by the strong continuum. This means the gas temperature is'only a little less than the electron temperature. Even if lowestpressures are used, only a small difference (about 150) exists betweenelectron and gas temperature; as measurements have shown. By comparison,in the case of a low-pressure discharge, the electron temperature ishigher than the temperature of the gas by a factor of 10 at least. Theaverage gas temperature in a wall-stabilized continuous burning xenonhigh-pressure discharge amounts to about 6500 to 9000 K. whereas theelectron temperature is about higher. Bycomparison, the difference in alow-pressure discharge would be 1000 to 10,000".

Consequently, lamps made according to the present invention represent atype of high-pressure discharge lamps which was unknown hitherto. Theahove,

E n( T)n)p ed where V is the volume of the total discharge envelope, (Vthe volume of the nth dead space, and the factor The factor a takes intoaccount the difference between the average temperature in the dischargeT and the gas temperature in the specific dead space (T (alltemperatures in K.).

With low filling pressures of less than 350 mm. Hg, preferably 20-200mm. Hg, used in these lamps, the operating pressure lies between about1/ and 4 atmospheres dependent upon the difference between space andplasma temperature as found from the temperature distribution calculatedper Elenbaas-Heller. With such low filling and operating pressures, thelamps are wallstabilized. In case of higher pressures, the discharge areis contracted; it no longer burns wall-stabilized but is subject toconvection disturbances. Generally, the mode of operation of the lampis, then, dependent on position. The upper pressure limit depends on thebulb diameter, and increases with decreasing diameter. The lower limitfor wall-stabilized discharges is determined by the requirement ofthermal balance in the plasma.

Within the pressure range used in lamps according to the presentinvention, under constant Wall loading and other constant conditions,the luminous efficiency is quite independent of pressure throughout awide range as experiments have shown. This follows from the fact ofincreasing plasma temperature with decreasing pressure previouslydescribed. Luminous eificiency would be improved thereby because thecontinuous radiation is almost independent of pressure over a widerange.

The luminous efiiciency with constant wall loading is in practice givenby the arc output per centimeter of length.

A certain arc output per centimeter may be obtained according to theabove explanations:

(1) in the usual manner by means of high pressures:

this entails high gradients, low current, small discharge cross section,high input concentration and stabilizing by additional means such asmagnetic fields, diaphragms -etc.; or

(2) as it is obtained in lamps according to the present invention, bymeans of low pressures: this entails low gradients, high current, largedischarge cross sections, low input concentration, and wallstabilization.

' Therefore, it follows from the present invention that .it} is notnecessary in high-pressure gaseous discharges to: use high pressures andouter stabilization. Nearly the same luminous efiiciency may be obtainedin oppositemanner, i.e., with low pressures and wall stabilizatron; insuch case however, contrary to higher pressure discharge, a highercurrent must be used and the arc diameter must be large.

A low filling pressure shows the advantage that in manufacture of thelamps, gas filling and sealing-01f the exhaust tube may be made alwayswith a pressure below atmospheric. By comparison with lamps usedhitherto, consumption of expensive filling gas is much smaller. lf lowpressures are used, particularly if the operating pressure in the lampis nearly equal to atmospheric, then the danger of breaking is,practically, eliminated and the wall thickness of the discharge tubemay, therefore, be kept small.

As the gas filling there may also be used instead of xenon one of theother rare gases, e.g., krypton, argon, neon or helium or mixtures ofseveral or of all rare gases. It is, however, known that the proportionof light radiation to heat losses is mostfavorable if xenon is used asthe filling gas whereby also wall loading is the least. Also additions,e.g., of hydrogen, CO metal vapours, halogens or nitrogen maybe presentin the rare gases.

The lamp operates especially favorably with regard to luminousefliciency at high wattages. Heat conduction takes place from the aresurface and is, at first, proportional to the surface area. If the arcdiameter increases, heat losses increase almostproportionally to thediameter; whereas the output, under the assumption of constant currentdensity and constant gradient, increases with the square of thediameter. Therefore as the diameter is increased, heat losses do notincrease proportionally to the wattage but only with the root of thewattage, Thus the proportion of radiation percentage to heat loss isshifted with increasing discharge diameter, thereby increasing wattagein favor of the radiation whereby an upper limit for the luminousefficiency is given by the plasma temperature. Tube diameters of thelamps according to the present invention are generally chosen not lessthan one centimeter, andin most cases greater. The cross section isdetermined by the current used for the discharge. Lamps according tothis invention may be manufactured for any high output and there is noupper limit in rating.

Lamps according to the invention may be operated with AC. as well aswith DC. The shape and size of the lamp electrodes are determined by thecurrent load and by the kind of operation, that is whether on DC. orA.C. There may, suitably, also be provided diaphragms, for instance inthe form of perforated quartz glass discs connected with the quartzglass envelope, in front of the electrodes as a protection againstsputtering. By suitably lengthening the discharge path, for instance to1 meter as it is possible in high-pressure discharge lamps according tothe invention, such a high operating voltage is obtained that theelectrode losses compared with the energy conversion in the arc becomeinsignificant, thereby favorably influencing efiiciency;

If compared with water-cooled lamps of similar output, lamps accordingto this invention show the advantages of simpler handling, lessneed ofattendance, and greater safety in consequence of omitting the outerenvelope and the water supplywith the pipes. The plasma temperature ofair-cooled, wall-stabilized lamps according to the present invention isjust slightly lower than that of liquid-cooled lamps.- Therefore, colortemperature and color appearance differ only little from sunlight, justas is the case with liquid-cooled lamps.

The limit for non-artificial cooling of lamps is given by the surfaceload capacity of the envelope. The abovementioned dataabout the maximumpermissible input concentration relates to quartz glass used as thematerial for the discharge tube. In case of transparent material whichmay'have a higher thermal load capacity than quartz glass, as forinstance aluminum oxide (sapphire), magnesium oxide etc., the inputconcentration in the discharge may be higher whereby luminous efiiciencywould .be furthermore increased. 5 Input concentration could, of

course, be further increased by using artificial cooling,

.xenonas the filling gas.

' e.g., by .blowing-in an air-current, by liquid-c ling r the Fig. 1showsa-lamp designed for A.C. operation. The

lamps may be operated because of their positive characteristic eitherimmediately on A.C. voltage of 220 volts or .on a higher voltage, ifdesired, by connecting in series a choke coil. The tubular .envelope 1made from quartz :glass and havingv an inside diameter d contains Acentral portion of the envelope has been broken .out to shorten thefigure. The cylindrical electrode bodies 2 and 3 of thoriated tungstentake about one third of the tube diameter. In front of the electrodesare mounted the perforated quartz glass discs 4 and 5 serving asprotective diaphragms. The electrodes are supported on conductors 6, 7sealed through tube extensions 8, 9 and connected to base terminals 10,11.

Fig. 2 shows a lamp designed for D.C. operation and which may also beoperated without series resistance. In this lamp the cathode 12 is madesmaller than the anode 13.

The following table I shows data on some wall-sta bilized xenonhigh-pressure discharge lamps without artificial cooling made accordingto the present invention and designated XBL 3000, XBL 16000, and XBL50000. The data are compared with corresponding data on a well knownwall-stabilized liquid cooled xenon long-arc lamp designated XBF 6001.

The above table shows clearly that the input concentration in w./crn. ofair-cooled lamps is smaller by one order of magnitude (factor of whencompared with that of liquid-cooled types. The input concentration ofthe above-mentioned air-cooled lamps lies below 100 w./cm. but that ofthe liquid-cooled XBF lamp amounts to more than 1000 w./cm. A preferredrange of input wattage concentration for xenon filled lamps in accordance with the invention is 10 to 80 watts per cubic centimeter. Theelectrical gradients and current densities of the air-cooled lamps arelow when compared with those of liquid-cooled lamps. Also the fillingpressure and, thereby, the operating pressure is low in the XBL lampsand the inside diameter of the discharge envelope is large by comparisonwith corresponding values of the XBF lamps.

The discharge tube of the lamp according to this invention is notrestricted to the straight form but it may also be bent in order to meetspecial optical requirements. There may be chosen beside the circulartube cross sections shown in the accompanying drawings any other crosssection form. It may be advantageous to operate A.C. lamps in threephaseconnection.

An additional increase in radiation in any desired spectral ranges maybe obtained by means of a combination with a known fluorescent materialor mixture. The intensity of light emission on one side of the dischargeenvelope may also be increased by providing a reflecting layer on a partof the discharge envelope.

Lamps as described in the present invention are quite suitable forillumination of large areas of all, kinds, asfor instance large rooms,railway yards, theaters, sport .fields and factories, forcoast lightingand landing ground illumination or the like.

The above described lamps proved particularly suitable for instance forfast color testing purposes as well as for aging plants. They areadvantageous also for blueprinting purposes because for such purposeslamps of great length and of large diameter are desired .in order tohave uniform illumination of large areas. The lamps may be used also inlarge radiation plants, e.g., for light baths, for cultivation of plantsor the like where all ranges of radiation may be utilized.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electric high pressure gaseous discharge lamp for continuousoperation comprising an elongated envelope of a radiation transmittingmaterial having a temperature resistance similar to quartz, a pair ofelec trodes mounted in said envelope at a distance apart amounting to atleast double the inside diameter of the envelope, and a gaseousionizable filling within said envelope of an inert gas from the groupconsisting of xenon, krypton, argon, neon, helium and mixtures thereofat a filling pressure equivalent to a pressure in the range of 5 to 350millimeters of mercury in a lamp without dead space, said lamp having aninput wattage material is quartz and wherein the input wattage con- Icentration in the discharge is in the range of approximately 10 to wattsper cubic centimeter.

3. A wall-stabilized electric high pressure gaseous discharge lamp asdefined in claim 1 wherein the equivalent filling pressure in a lampwithout dead space lies between 20 and 200 millimeters of mercury.

4. A wall-stabilized electric high pressure gaseous discharge lamp asdefined in claim 1 wherein the rare gas filling contains a minorproportion of an additional gas from the group consisting of carbondioxide, metal VEfIPOI'S, halogens, hydrogen, nitrogen and mixturesthere- 0 5. A wall-stabilized electric high pressure gaseous dischargelamp as defined in claim 1 whereof the envelope consists of a radiationtransmitting material having a higher temperature resistance thanquartz.

6. An electric high pressure gaseous discharge lamp for continuousoperation comprising an elongated tubular quartz envelope, a pair ofelectrodes mounted in said envelope at a distance apart amounting to atleast double the inside diameter of the envelope, and a xenon fillingwithin said envelope at a filling pressure equivalent to a pressure inthe range of 20 to 200 millimeters of mercury in a lamp without deadspace, said lamp having an input wattage concentration in the range ofapproximately 10 to 80 Watts per cubic centimeter and being soproportioned with respect to distance apart of the electrodes and insidediameter of the envelope that with the said input concentration in theabsence of any artificial cooling a wall-stabilized discharge isachieved.

7. A wall-stabilized electric high pressure gaseous discharge lamp asdefined in claim 6 wherein said inside envelope diameter is not lessthan approximately 1 centimeter.

8. An electric high pressure gaseous discharge lamp for continuousoperation comprising an elongated tubular envelope of a radiationtransmitting material having a temperature resistance similar to quartz,a pair of electrodes mounted in said envelope at a distance apartamounting to at least double the inside diameter of the envelope, and agaseous ionizable filling within said envelope of .an inert gas from thegroup consisting of xenon, krypton, argon, neon, helium and mixturesthereof at a filling pressure equivalent to a pressure in the range of 5to 350 millimeters of mercury in a lamp without dead space said lamphaving an input wattage concentration up to 1000 watts per cubiccentimeter and being so proportioned with respect to distance apart ofthe electrodes and inside'diamet'er of the envelope that with the saidinput concentration and with the said filling pressure in the presenceof artificial cooling'a wall-stabilized discharge is achieved.

References Cited in' the file of this patent UNITED STATES PATENTS2,298,239 Stirnk orb Oct. 6, 1942 2,367,595 Marden Jan.- 16, 19452,654,043 Freeman et al. Sept. 29, 1953 2,761,086 Noel et al. Aug. 28,1956 2,774,013

. Willoughby Dec. 11, 1956

1. AN ELECTRIC HIGH PRESSURE GASEOUS DISCHARGE LAMP FOR CONTINUOUSOPERATION COMPRISING AN ELONGATED ENVELOPE OF A RADIATION TRANSMITTINGMATERIAL HAVING A TEMPERATURE RESISTANCE SIMILAR TO QUARTS, A PAIR OFELECTRODES MOUNTED IN SAID ENVELOPE AT A DISTANCE APART AMOUNTING TO ATLEAST DOUBLE THE INSIDE DIAMETER OF THE ENVELOPE, AND A GASEOUSIONIZABLE FILLING WITHIN SAID ENVELOPE OF AN INERT GAS FROM THE GROUPCONSISTING OF XENON, KRYPTON, ARGON, NEON, HELIUM AND MIXTURES THEREOFAT A FILLING PRESSURE EQUIVALENT TO A PRESSURE IN THE RANGE OF 5 TO 350MILLIMETERS OF MERCURY IN A LAMP WITHOUT DEAD SPACE, SAID LAMP HAVING ANINPUT WATTAGE CONCENTRATION IN THE RANGE OF 5 TO 200 WATTS PER CUBICCENTIMETER AND BEI-NG SO PROPORTIONED WITH RESPECT TO DISTANCE APART OFTHE ELECTRODES AND INSIDE DIAMETER OF THE EVELOPE THAT WITH THE SAIDINPUT CONCENTRATION AND WITH THE SAID FILLING PRESSURE IN THE ABSENCE OFANY ARTIFICIAL COOLING A WALL-STABILIZED DISCHARGE IS ACHIEVED.